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A/C Usage

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  • A/C Usage

    I'm hoping someone can help me. I have been searching the internet looking for an article I read a few years ago about A/C usage. It was an article about how energy is actually wasted by turning up the A/C thermostat or completely off when not at the house/facility, and it is actually better to turn the temp down a little overnight with an A/C system, since evening energy is cheaper than peak hour energy. Our maintenance guys are freaking out because the chiller is about ready to blow a gasket running like it is each day.
    Our work just hired an "energy czar" and they have begun completely shutting off the A/C chiller at 5 PM each night. It is set to start up at 7 am each morning, but with the recent heat, it has been any where from 84 to 90 in the labs when we come in. A lot of our equipment has temperature and humidity sensors on them, and they will not work if either is too high.
    If anyone can help, it would be much appreciated.

  • #2
    Re: A/C Usage

    There is no one simple answer, and the best place to start looking is your energy utility company. They have a lot of data.

    First, you would have to know whether you are actually billed based on time of day usage.

    Second, someone would have to analyze the building. There probably is a "break even" number, where it costs less to maintain a certain nominal temp, than to let it warm up completely then have to cool down. This would be a function of building characteristics and A/C system specs, seer rating, etc.

    Try this: it is a new "smart" search engine. I have not tried it much yet, but you are supposed to be able to "ask questions"


    • #3
      Re: A/C Usage

      Relitive humidity will kill you overnight. If that rises too high, the unit will have to run X hours just to reduce the latent load and THEN you'll see a sensible drop in temperature. There is a point at which you'll waste energy but OFF is never a good idea. I'll even tell my clients (residential mind you) to leave it at 80-83 even while away on vacation. Aside from the work to cool the space down, there is also an issue with mold and mildew in high humidity areas.
      Sorry I can't help with specifics but it's without a doubt not good to shut it down all night.


      • #4
        Re: A/C Usage

        Most commercial customers that are high demand electrical users are billed differently then most...

        Customers base rate (cost per kilowatt) is set based on peak usage. So the key is to keep your total peak down... During daytime hours and high temperatures your peak is naturally highest and this is where the bar is set for most.

        You can see a huge savings by running your high load items (chillers) during of peak times and banking your cooling (Ice banking) for use during peak times. (This is where energy managers make the bucks)

        This practice will lower your per kilowatt cost by lowering your peak demand usage for every kilowatt you use.

        This practice will also lower the demand on your servicing grid prompting further discounts from your electricity provider.

        If you do not have the ability to bank your cooling capacity this is a moot point though... The next best thing is to limit usage as best as possible.

        Turning off the chiller after business hours is a best practice if used in conjunction with a properly set up energy management system and the system is designed accordingly. If management is just saying we need to save a few bucks so lets turn off the chiller at night this is a very bad idea. If it is not set up properly you will have alot of ticked off uncomfortable customers...

        Once you get behind on a load it is not a quick process to catch up, we have had facilities that would take days to catch up from 4 hour outages. Daily shut off will not be effective on older systems ( older systems were designed to be run) and you will lag the load.

        It sounds like this comes down to a $$$ decision over peoples comfort so be ready for a hot summer @ work


        • #5
          Re: A/C Usage

          Monday morning at 7 am it was 89 degrees. It went down to about 84 by Tuesday morning, and 81 Wednesday morning. This morning it is 73, but is is pretty chilly outside. You can tell the humidity is up in the building, even at 73.
          The building was built in 1968, it is an industrial plant without the A/C and a large lab/office area. All the ducting and piping is still original to the building. I believe the chiller internals have been updated, but the air handling system has not been modified, just fixed when broken. Trane is here all the time doing something. I doubt the system is set up to maximize energy usage, but they did rework the zone controls 2 years ago to even out the temperature in the building, but it doesn't work too well. I guess when the chiller blows, then someone will give them some data to set them straight. Thanks for the replies.
          Last edited by nkyrental; 06-04-2009, 06:50 AM.


          • #6
            Re: A/C Usage

            If your lab equipment has to be maintained at a specified temperature and relative humidity the labs should be conditioned on a seperate system that can be run all the time. The rest of the facility can be maintained at whatever your management/energy czar desires.